Methods have been developed for quantifying messenger RNA and for efficient cloning of cDNA. (1) The mole percent polyadenylated RNA in a preparation contaminated with rRNA has been determined by labeling with poly(A) polymerase and cleaving the product with ribonuclease H in the presence of oligothymidylate. The absolute concentration of submicrogram quantities of mRNA can also be ascertained. (2) Single stranded cDNA has been prepared with reverse transcriptase in 400% yield by modifying the template RNA. (3) Double stranded cDNA has been synthesized in 70 to 80% yield by priming the polymerization of the second strand with RNA fragments generated by the ribonuclease H associated with reverse transcriptase. (4) The conditions under which 3-feet-ends of cDNAs are extended with homopolymeric DNA have been altered. Tails of uniform length at both ends of each molecule have been synthesized in virtually 100% yield. (5) Cerenkov radiation has been used to monitor reactions performed in small volumes and to quantify precious biological materials with losses of less than 0.5 Mul.